As electronic and wireless technology becomes more widely used, it becomes more important to protect our audio signals from the electromagnetic noise all around us. Balanced cables are a great tool to combat the ever-increasing threat of electromagnetic interference.
A balanced cable is a cable which preserves the quality of an audio signal and cancels noise that enters the signal path. They are particularly useful for long distances. However, as a best practice, balanced cables should be used whenever possible. In this post, we’ll take a look at the technique balanced cables use to do this and the different types of balanced cables used in professional audio.
A balanced cable is a cable which preserves the quality of an audio signal and cancels noise that enters the signal path.
The Purpose of Balanced Signals
The electrical signal produced by a microphone is generally very low level, or quiet. Signal coming from a microphone is amplified by a microphone preamp. This means that if any noise enters the signal path, the noise will be amplified, too. How does noise enter the signal path?
Electromagnetism is the interaction between electric currents and magnetic fields.
If a magnetic field intersects a cable, an electrical current is created on the cable. Inversely, if an electrical current is sent through a cable, a magnetic field forms around the cable.
As mentioned above, electromagnetic waves are all around us. They come from a variety of sources. Every wireless device and every electric current moving through a cable radiates electromagnetic energy. You can see an illustration of electromagnetism in the image below. The line along the cable represents the magnitude of the electrical current and the rings represent the magnetic field created by the electric current.
If a magnetic field passes by a cable, some of the energy will be transferred to the cable. This is called induction.
Remember that every cable carrying alternating electrical current creates a magnetic field. This means that if two cables run parallel to one another, the signal from one can jump to the other.
You can see induction illustrated below. The signal from one cable radiates outward and is superimposed onto the other cable.
Although there are electromagnetic waves all around us, audio engineers should be particularly worried about the waves of energy that are within the frequency range of human hearing. These are the frequencies that will cause audible noise in our audio.
The frequency of most waves in the air is far greater than 20 kHz, the upper limit of human hearing. These waves won’t cause audible noise in our audio because they are too high for us to hear. Many frequencies are in the audible range, however. What are the most common noises superimposed into audio signals?
Hum – 60 Hz Power
In the United States, the standard frequency for power distribution to buildings is 60 Hz. In some other parts of the world, 50 Hz is the standard. We have all heard the sound of hum from a power line. Turn your speakers up loud enough, and you’re bound to hear the low hum in the background.
Everywhere you look, you see electronic devices.
Any device which uses power from the wall emits an electromagnetic wave at 60 Hz. Every cable that plugs into a wall outlet emits an electromagnetic wave at 60 Hz. It’s impossible to eliminate the noise emitted by our power systems, so it’s important to use the correct tools to protect our signals from this noise.
Buzz – Poor Grounding
Buzzing in an audio signal often comes from power lines, just like hum. Although power oscillates at 60 Hz, the harmonics of 60 Hz can also be heard. Harmonics are the overtones of a frequency.
Harmonic overtones are the fundamental frequency added to itself again and again. In our power example, the fundamental frequency is 60 Hz. Its first harmonic is 120 Hz (60 + 60). Its second harmonic is 180 Hz (60 + 60 + 60). This sequence continues, adding the fundamental frequency each time: 60 Hz, 120 Hz, 180 Hz, 240 Hz, and so on. Buzz is usually a harmonic of 60 Hz, caused by electronic devices.
Buzzing is most often caused by poor grounding. We will get into the practices of proper grounding in a different post. The takeaway here is that most of the noise superimposed onto our audio signals are a result of power.
Cell Phone Interference
Have you ever set a cell phone on a guitar amplifier or a speaker and heard the “blip, blip, blip, buzz…” sound? This is called GSM interference and it’s caused by the high-powered electromagnetic output of a cell phone transmitter.
When data is being transferred between a cell phone and a tower, some of the energy from the waves carrying the data is superimposed, or induced, into the audio signal.
Cell phones and wireless devices are becoming more and more ubiquitous. Thus, it is more important than ever to do what we can to protect our audio with proper cabling.
What Is The Difference Between Balanced and Unbalanced Circuits?
The primary difference between a balanced cable and an unbalanced cable is the number of conductors, or wires, running through it. A balanced cable contains 2 wires and a shield, while an unbalanced cable contains only 1 wire and a shield. Let’s take a deeper look into these two types of cables.
An unbalanced audio cable consists of one wire and a shield.
The signal wire carries the audio signal from the source to the destination, while the shield acts as a return to complete the circuit.
The shield is generally wrapped around the signal wire throughout the cable, in an attempt to intercept some electromagnetic energy that would interfere with the signal. However, this is often not enough to protect audio signals completely.
Although the shield protects the signal wire somewhat, electrical currents are still superimposed onto the signal wire from electromagnetic waves passing by. Take a look at the image below. Notice that there is only one copy of the audio signal travelling from the source to the destination.
A balanced cable contains two wires and a shield. What sets a balanced cable apart from an unbalanced cable is the way it uses these components. Let’s first take a moment to make sure we have a good understanding of phase.
If two signals are perfectly in phase, they will add together. The result will be a signal that is the sum of each.
It’s like 1 + 1 = 2. Here is what it would look like on a graph:
If two signals are perfectly out of phase, or 180-degrees out of phase, they will cancel each other out. The result will be no signal at all.
It’s like 1 + -1 = 0. Here is what it would look like:
How Balanced Cables Work
The first wire in a balanced cable carries the signal. The second wire carries the signal, too. However, before the signal is sent down the second wire, the polarity is flipped. This means that the two signals are 180-degrees out of phase, and therefore complete opposites.
You can see this in the image below. (Note: The wires are twisted in a real cable.)
As the two copies of the signal travel along the cable, they are both susceptible to electromagnetic noise.
However, although the audio signals are complete opposites of one another, the noise induced into the wires will be identical in both wires.
You can see this in the image below. The red line represents the noise induced into both wires in the cable.
Here’s where it gets interesting…
When the signals reach the destination, the signal on the second wire is flipped again, putting it back into phase with the first wire. Now the two copies of the audio signal are the same, and the noise is opposite on each wire. Now, the audio signals sum together, while the noise cancels out.
In a balanced circuit, the audio signals that are opposite on the two wires as they travel down the cable sum when flipped back into phase at the destination. This is called differential mode gain. The noise that is identical on each wire cancels when flipped at the destination. This is called common mode rejection.
So, not only did we cancel out any noise induced into the cable, but we also doubled the strength of our signal by adding the two wires together.
Types of Connectors
There are many audio connectors out there. Which connectors are balanced, and which are not? How can you tell the difference? Generally speaking, if a mono cable has 3 pins, it is balanced. If a mono cable has only 2 pins, it is unbalanced.
Quarter-inch connectors do not have pins. They instead have terminals called tips, rings, and sleeves. The various types of ¼” connectors are named according to the terminals they contain.
For example, if a connector has only a tip and a sleeve, it is a TS connector. If it has a tip, a ring, and a sleeve, it is a TRS connector.
¼-Inch TS Connectors
A quarter-inch TS connector has a tip and a sleeve.
Cables with these connectors are often called instrument cables, as they are the standard for guitars, keyboards, and other musical instruments.
This connector is only for unbalanced signals.
The tip connects to the signal wire (+) and the sleeve connects to the shield, or return wire.
¼-Inch TRS Connectors
A quarter-inch TRS connector has a tip, a ring, and a sleeve.
These cables are balanced and are used for line-level signals.
The tip connects to the signal plus wire (+), the ring connects to the signal minus (-) wire, and the sleeve connects to the ground, or shield wire. Many audio consoles, or mixers, use TRS jacks for balanced connections because they take up less space than an XLR jack.
3.5mm or ⅛-inch Connectors
3.5mm, or ⅛-inch, connectors are usually used for consumer electronics. This is the standard for headphone connections on modern devices.
They also use the tip, ring, sleeve naming convention.
3.5mm TS Connectors
A 3.5mm, or ⅛-inch, TS connector has a tip and a sleeve. These connectors are unbalanced and are used for a variety of consumer applications.
The tip connects to the signal wire (+) and the sleeve connects to the return. Some inexpensive microphones use a 3.5mm TS connector so that they can be plugged into a laptop line-level input.
3.5mm TRS Connectors
A 3.5mm, or ⅛-inch, TRS connector has a tip, a ring, and a sleeve. Although these connectors have 3 terminals, they are usually unbalanced stereo cables.
The tip connects to the left channel (+), the ring connects to the right channel (+), and the sleeve connects to the return. This is the standard connector used for modern headphones. Cables with this connector are sometimes called “Aux Cables” because they are used to connect auxiliary sources, such as a smartphone, into car or home stereos.
3.5mm TRRS Connectors
A 3.5mm, or ⅛-inch, TRRS connector has a tip, two rings, and a sleeve. They are unbalanced connectors usually found on headsets, which contain a microphone and two headphones.
The tip connects to the left channel (+), the first ring connects to the right channel (+), the second ring connects to the microphone (+), and the sleeve connects to the return.
RCA connectors are unbalanced connectors used for consumer electronics.
They are usually paired together as white (left) and red (right). The pin in the center connects to the signal wire (+) and the ring connects to the return.
The most common professional audio connector is XLR. These connectors have 3 pins.
The first pin is plus (+), the second pin is minus (-), and the third pin is ground, or shield.
While an XLR-to-XLR cable is for balanced signals, XLR adapters are usually not.
For example, a 3.5mm-to-XLR cable is unbalanced. This might be confusing, because stereo 3.5mm connectors have 3 pins. However, the 3 pins of a 3.5mm connector are used for left, right, and return. There are two unbalanced circuits running in the same cable. In order for a circuit to be balanced, all components in the circuit must be balanced.
Protect Your Audio
If you understand what causes noise, you can understand how to avoid it. Use balanced connections whenever possible and keep your cables away from sources of electromagnetic noise.
Once noise is recorded, it is impossible to remove without damaging your audio signal. It’s better to use best practices to avoid the noise altogether.